In recent years, handheld terminal devices such as laptop personal computers, cellular phones, and PDAs (Personal Digital Assistants) are being remarkably widespread. As the secondary batteries as the power source in these handheld terminal devices, e.g., nickel-hydrogen secondary batteries and lithium ion secondary batteries are often used. The handheld terminal devices are required to have a comfortable portability, and such devices are rapidly becoming more compact, thin and lightweight with better performance. As a result, the handheld terminal devices are now being used in a wide variety of situations. Like the handheld terminal devices, the secondary battery is also required to be more compact, thin and lightweight with better performance.
For improving the performance of the secondary battery, there have been studied modification of the electrode, the electrolyte liquid, and other members of the battery. Among them, the electrode is usually produced by: mixing an electrode active material and, if necessary, a electroconducting agent such as electroconductive carbon, with a liquid composition in which a polymer serving as a binder (binding agent) is dispersed or dissolved in a solvent such as water or an organic liquid to prepare a slurry composition; then applying the slurry composition onto a current collector; and drying the slurry composition. As to electrodes, in addition to the studies on the electrode active material and current collectors themselves, there have also been made studies on binders for binding electrode active materials, etc. to a current collector, as well as a variety of types of additives (see, e.g., Patent Literatures 1 to 4).
For example, Patent Literature 1 discloses use of a binder containing crosslinkable polymer particles. Patent Literatures 2 and 3 disclose a slurry for a negative electrode of a non-aqueous secondary battery, the slurry containing a binder composed of a carboneous active material, a water-dispersed emulsion resin, and a water-soluble polymer. As the water-soluble polymers, those such as polyvinyl alcohol, carboxymethyl cellulose, and sodium polyacrylate are described. According to the description of the literatures, the coating strength and coating density in the batteries can thereby be made favorable.
Patent Literature 4 discloses a binder for an electrode of a secondary battery. The binder in this Literature consists of a copolymer latex obtained by emulsion polymerization of monomers composed of 0.02 to 13% by weight of a fluorine-containing unsaturated monomer, 10 to 38% by weight of an aliphatic conjugated diene monomer, 0.1 to 10% by weight of an ethylenically unsaturated carboxylic acid monomer, and 49 to 88.88% by weight of a monomer copolymerizable with the aforementioned monomers. According to the description of the literature, this results in good properties regarding mixing stability, blocking resistance, anti powder-falling property and binding strength.
Further, Patent Literature 5 discloses a binder for a secondary battery electrode. The binder in this literature is composed of a polymer with a monomer unit derived from a fluorine-atom containing monomer such as a fluorinated alkyl (meth)acrylate. The literature also discloses that a cellulose-based polymer, a polyacrylate, and the like may be added in order to improve the application capability and the charging-discharging property. According to the descriptio of the literature, an electrode which can persistently exert high bonding property with an active material can thereby be obtained.